From owner-chemistry@ccl.net Thu Sep 22 14:08:00 2011 From: "Patrick Bultinck Patrick.Bultinck%ugent.be" To: CCL Subject: CCL: G09: Convergence Issues for H-Bonded Systems Message-Id: <-45496-110922134807-18589-0+JoP4+mMH46tBI8hzVvvw:server.ccl.net> X-Original-From: Patrick Bultinck Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=us-ascii Date: Thu, 22 Sep 2011 19:47:46 +0200 Mime-Version: 1.0 (iPhone Mail 8L1) Sent to CCL by: Patrick Bultinck [Patrick.Bultinck#,#ugent.be] Just as a minor comment: drop that basis set. It does not render benzene planar for MP2, so I scare to think what it might be up to now. Second, I would get the Hessian from the fchk file, diagonalize that myself, see what that does and then compute the displacement eigenvector for the lowest eigenvalue and displace the geometry for minimal displacements along this vector and repeat the procedure. You could even think of some sort of grid of displacements and see if you could do some sort of line search along that. A near zero eigenvalue means that the energy does not change along that vector. Think of translation/rotation of the molecule as a whole. Check what your vector is like. And maybe it's just a zero direction: you may be looking at nice chemistry! Or some fragment may be so far out that you're seeing free movement. A minimal amount of algebra will tell. Patrick On 22 Sep 2011, at 17:25, "Sam Abrash sabrash]_[richmond.edu" wrote: > > Sent to CCL by: "Sam Abrash" [sabrash_+_richmond.edu] > Hi Folks, > > I have a job involving hydrogen bonding of 3 acetylenes to the benzene cation. I'm having trouble with the geometric convergence. I was able to get the system to converge with normal convergence criteria, opt=calcall, and MaxStep=5, but then I got an imaginary frequency of -1.9 wavenumbers. > > Subsequently, I changed to Opt=tight, tried both calcfc and calcall, and have steadily reduced MaxStep to 1, but it still won't converge. The force constant criteria have been met, but the problem is the displacement criteria. Both this observation and looking at the structures with JMOL show that the problem is finding the minimum in a very shallow potential. > > Two questions. First, is the 1.9 wavenumber imaginary frequency real or an artifact? Second, any advice on how to force the system to converge? Model chemistry is M062X/6-311++G**. > > Thanks! > Sam> >